In recent years, wetness indicators have come to be commonly used on absorbent articles, such as baby diapers, adult incontinence pads, and training pants, as a visual indication that the absorbent article is wet with urine. These wetness indicators generally fall into two types. One type, a dissolvable ink, is an ink which is visible while the absorbent article is dry but dissolves and becomes invisible when the absorbent article becomes wet. Another type is an ink that is either invisible or a first color when dry, but which becomes visible or changes to a second color when urine contacts the ink. Generally, these types of ink are considered non-dissolvable inks. In either case, the urine must contact the ink directly in order for the wetness indicator to perform correctly.
The purpose of a wetness indicator is to provide visual information to a caregiver that the absorbent article is wet, without the caregiver having to unfasten or remove the absorbent article. Therefore, to function correctly, the wetness indicator must be visible on the outside of the absorbent article (also called the garment side or garment-facing side). Therefore, the wetness indicator ink should be applied to the outer cover of the absorbent article, in order to be visible to a caregiver.
However, the primary function of the outer cover of an absorbent article is to act as a moisture barrier, preventing urine or other body fluids from escaping the absorbent article. Any absorbent article with an outer cover that permitted body fluids to leak out would be completely unacceptable to consumers. The outer covers of disposable absorbent articles are usually made of polymer films, which are excellent fluid barriers.
This presents the absorbent article manufacturer with a conundrum. To provide the optimal visibility, the wetness indicator ink should be on the outer cover of the absorbent article. To act as a moisture barrier, the outer cover should not allow liquids to leak out. However, to provide the caregiver with the desired visual cue, the aqueous liquid, e.g., urine, needs to directly contact the wetness indicator ink.
The previous solution to the conundrum has been to apply wetness indicator inks to the interior surface (i.e. body-facing or ‘wet’ side) of the polymer film that comprises an absorbent article's outer cover. This way, the urine can contact the wetness indicator ink inside the absorbent article, and the visual cue can be seen on the outside of the absorbent article without liquid leakage through the polymer film.
The polymer films used as liquid barriers in outer covers can retain more than liquid. For example, these films can also retain body heat, perspiration and moisture vapor, which can make the absorbent article hot, steamy and uncomfortable. To counter this discomfort, many absorbent article backsheets are made from microporous breathable polymer films. These films, which are well known in the art, are typically made from polymer resin that is filled with finely-divided particles of inorganic material such as calcium carbonate. A film made from this filled polymer is then stretched to cavitate the polymeric matrix around the filler particles, which creates micropores that form tortuous paths through the film. Molecules of water vapor can escape the absorbent article interior by travelling through the tortuous paths of micropores, which means that the article is more comfortable to wear.
But microporous films are also opaque or semi-opaque, because the myriad internal surfaces created within the film by the micropores reflect ambient light. Often, this opacity is a desirable property as it helps to mask the presence of urine, feces, or other body fluids within the absorbent article. However, this opacity also masks the appearance of wetness indicator inks that are printed on the interior surface of the outer cover film. Because of this masking effect, manufacturers must print wetness indicator markings that are dark or intensely colored, with heavy ink coverage, so the wetness indicator can be seen through the opaque outer cover film. In some cases, the film may be made with a lower opacity to allow the wetness indicator ink to show through, but this decrease in opacity also means the masking power of the film is reduced, potentially allowing the urine or feces to be visible through the film.
The heavy application of wetness indicator inks can lead to another problem. For example, with baby diapers, it has been found that these inks can sometimes dissolve in the baby's urine, then wick back through the absorbent core and transfer onto the baby's skin. This is both unsightly and creates an unnecessary exposure of the baby's skin to the ink components.
Finally, most disposable absorbent article outer covers are now printed with graphics that decorate and enhance its appearance. Because of the opacity problem already discussed, and to achieve the best appearance, these graphics are usually printed on the exterior (garment-facing side or ‘dry’ side) of the outer cover film. However, the wetness-indicator graphics have historically been printed on the interior side of the film. Therefore, the manufacturer must print graphics on both sides of the outer cover film, which can be costly. It is even more complex and costly to print the wetness indicator ink on the inside and the main diaper graphics on the outside in register with each other to achieve visually striking effects.
In view of the foregoing, a need exists for wetness-indicating polymer films having wetness-indicator (WI) inks printed on the exterior or ‘dry’ side of an absorbent outer cover (backsheet) film.